Who says? Actual prospect industry scouts, experts, and evaluators...You know, people who do this for a living. As @sveumrules pointed out, Chourio and Tatis Jr. are/were top 5 multitool teenage prospects, while Singleton was a hit-only prospect who peaked at #25. Even then, Chourio's age 18 season with the bat was significantly better than Singleton's age 18 season with the bat.
Also, the DSL, while technically "rookie ball", is in practice a level beneath the Brewers' Arizona rookie affiliates. Henry Mendez, for instance, was in the same international FA class as Chourio, but still did the DSL AND ACL, while Chourio skipped the ACL entirely and then progressed all the way to AA. But, yes, 25 years and Prince Fielder are more relevant for thinking about how the Brewers will handle Chourio's progression...